Virtual image display device

ABSTRACT

An intermediate image is formed inside the prism by a projection lens or the like, and since an image light that is totally reflected at three or more surfaces; a third surface firstly, a first surface firstly, at a third surface secondly, at a first surface secondly, and the second surface in this order, transmits the first surface and then arrives at an observer&#39;s eyes, the thickness of the prism can be reduced and the size and weight of the entire optical system can be reduced, and it is possible to realize a display of high performance and brightness with a wide angle of view. Since the external light can be observed after passing through the first surface and the third surface, and the visibility at that time becomes substantially zero, it is possible to reduce the defocus or distortion of the external light when observing the external light.

BACKGROUND

1. Technical Field

The present invention relates to a virtual image display device thatprovides an image formed by an image display element or the like to anobserver, particularly to a virtual image display device that issuitable for a head-mounted display mounted on the observer's head.

2. Related Art

A variety of optical systems are proposed, which are incorporated into avirtual image display device such as a head mount display (hereafter,referred to as HMD) that is mounted on an observer's head (refer toJapanese Patent No. 2746697, Japanese Patent No. 3787399, JapanesePatent No. 4218553, and Japanese Patent No. 4819532).

Regarding the virtual image display device such as the HMD, it isdesirable to widen an angle of view and to reduce a weight of thedevice. Particularly, in order to improve a wear, it is important tomake the device thin in thickness in a viewing axis direction of theobserver and to make the center of gravity of the device closer to theobserver.

In addition, if a view of the observer is entirely covered such that theobserver can see only the image light, the observer cannot see theoutside state and may have a feeling of anxiety. Furthermore, a varietyof new applications of the virtual reality can be generated by making itpossible to see the outside and the image in overlapping. For thisreason, a display that displays the image light in overlapping withoutinterfering with the view of the outside is desired.

Furthermore, in order to improve the wear on the observer and to make anexternal appearance of the device better, it is generally desirable totake on the form of glasses in general, and to place the image displaydevice beside the face, not over the eyes.

In order to separate the image display device from the position of theeyes of the observer such that the size of the optical system becomessmall and the view is not interfered, it is preferable to use a relayoptical system in which an intermediate image is formed after forming animage of the display image light in the optical system once, and inwhich the enlarged intermediate image is displayed.

For example, in Japanese Patent No. 2746697, as a relay optical system,an optical system is proposed, in which an intermediate image is formedinside of a light guide plate using the light guide plate ofplane-parallel shape in which the end surface has a parabolic mirror anda projection lens. However, in a case of the optical system in JapanesePatent No. 2746697, since the projection lens is large, it is difficultto reduce the size and weight.

In Japanese Patent No. 3787399, as a relay optical system, an opticalsystem that includes a light guide prism having an emission reflectionsurface of curved surface and a projection lens, is proposed. However,in the optical system in Japanese Patent No. 3787399, there is noconsideration for the outside viewing by the observer. In order for theobserver to see the outside using this optical system, it is needed topaste a compensation prism on the reflective surface spread over theentire prism and to provide a half mirror on the bonded surface. Then,the image light is reflected twice on the half mirror surface, and thusthe image becomes very dark.

In Japanese Patent No. 4218553, as a relay optical system, an opticalsystem that includes a projection lens, a concave mirror, and a lightguide plate is proposed. In the optical system, the reflectionefficiency is increased by combining a wavelength plate and apolarization half mirror. However, in order for the observer to view theoutside using the optical system in Japanese Patent No. 4218553, it isneeded to paste a compensation lens together with the concave mirror,and accordingly, the system becomes thick as a whole.

In Japanese Patent No. 4819532, as a relay optical system, an opticalsystem in which the overall length is shortened to be compact by bendingthe light path is proposed. However, in a case of the optical system inJapanese Patent No. 4819532, a convex portion of the prism center and aprojection lens interfere with the view. In addition, the image light isfolded back after transmitting through the half mirror, and is reflectedat the half mirror, and is incident on the eyes. Therefore, the observedimage becomes dark.

Moreover, in a method of forming a virtual image using a light guideprism, for example, bending of the light path on a numbers of differentcurved surfaces by forming a light guide prism by connecting a pluralityof prism portions or forming a light guide prism by dividing a lightguide prism into a plurality of prism portions, can be considered. Inthis case, the degree of freedom is increased in each curved surface,and an improvement in image forming performance is expected. Incontrast, depending on the optical design, a use environment, and thelike, there is a concern that the external light is reflected in anunintended direction, and a ghost light can be generated in the cuts andthe joints between the curved surfaces.

SUMMARY

An advantage of some aspects of the invention is to provide a virtualimage display device that has a see through function with which anexternal light and an image light are displayed in overlapping, and hasa wide angle of view and a high performance in function, and isminimized in size and weight.

An aspect of the invention is directed to a virtual image display devicefor recognizing an image light and an external light at the same time,the device includes: an image element that generates an image light; anda prism that includes three or more non-axisymmetric curved surfaces,and in which an intermediate image is formed as a part of an opticalsystem. When the external light passes through a first surface and athird surface among a plurality of surfaces that configures the prism torecognize the outside, the visibility is substantially zero. The firstsurface and the third surface form a concave surface shape with respectto an observer side. The image light from the image element is totallyreflected at the third surface, totally reflected at the first surface,totally reflected again at the third surface, totally reflected again atthe first surface, and reflected at the second surface, and thentransmits through the first surface, and arrives at the observer side.Here, the image light is a light that is formed by an image element orthe like and can be recognized by eyes as a virtual image, and is formedas an intermediate image inside the prism.

In the virtual display device, an intermediate image is formed insidethe prism by the optical system. Furthermore, since the image light isreflected at the third surface, the first surface, the third surface,the first surface, and the second surface in this order, and then ittransmits through the first surface and then arrives at the observer,the prism can be made thin and the size and weight of the entire opticalsystem can be reduced, and thus, it is possible to realize a displaywith high performance and brightness with a wide angle of view. Inaddition, the external light can be observed after passing through thefirst surface and the third surface, and the visibility at that timebecomes substantially zero. Therefore, it is possible to reduce thedefocus or distortion of the external light when observing the externallight in see-through. In addition, the shape of the prism is made alongthe face of the observer, the center of gravity can be close to theface, and the design can also be made excellent. In addition, since theprism is configured where the total reflection is performed multipletimes on the first surface and the third surface respectively, thegeneration of discontinuous part between the surfaces along a lightguide path of the prism can be reduced. As a result, the occurrence ofsuch a situation that the ghost caused by the unintended reflection orthe like at the discontinuous part of the prism can be avoided.

In a specific aspect of the invention, in the virtual display device,with an origin of each surface which configures the optical system to bea reference, when an expression of a surface shape is polynomiallyexpanded with respect to an orthogonal coordinates x and y which isextended in a tangential direction from the origin, then conditions inthe below-described expressions 1 to 3 are satisfied with coefficientsof the terms x^(m)·y^(n) of the polynomial which indicates the k_(th)surface as Ak_(m,n).

−5×10⁻² <A1_(2,0) +A1_(0,2)<−1×10⁻³ and −5×10⁻² <A3_(2,0)+A3_(0,2)<−1×10⁻³  (1)

|A3_(2,0) −A3_(0,2)|<5×10⁻²  (2)

|A1_(2,0) −A3_(2,0)|<5×10⁻³ and |A1_(0,2) −A3_(0,2)|<5×10⁻³  (3)

Here, in the above description, the local coordinate (x, y, z) thatincludes the orthogonal coordinate x and y of each surface has an originon one point on the curved surface, and has a z axis in a tangentialdirection of the surface, an x axis and a y axis in a normal directionof the surface. The origin of the curved surface, for example, is theposition where the center of the light beam passes, and all of theorigins are located on the same surface (reference surface).

In the specific aspect of the invention, it is possible to observe bothof the image light and the external light that makes both of the firstsurface and third surface to be a concave surface shape toward theobserver. Additionally, by forming a free-curved surface with the firstsurface and the third surface, and effectively using the degree offreedom of the curved surface shape, it is successful to obtain anoptical system with a high image quality. A function of the firstsurface and the third surface, that is, a factor that characterizes thefunction of the curved surface is a curvature of the curved surface.Since the curvature near the origin is mainly determined by values ofcoefficients Ak_(2,0) and Ak_(0,2) (k=1, 3), it is important to set thevalues of the coefficient Ak_(2,0) and Ak_(0,2) appropriately.

The condition (1) defines the magnitudes of the curvature of the firstsurface and the curvature of the third surface near the origin. If thevalues A1_(2,0), A1_(0,2), A3_(2,0), A3_(0,2) are negative values, theyrepresent the fact that the first surface or the third surface has aconcave surface shape with respect to the observer. When the calculatedvalues exceed the upper limit of the condition (1), the shape becomesclose to a plane, even though the problem of the external light to theobserver does not exist, the aberration correction of the image lightdoes not effectively function. In addition, when the calculated valuesexceed the lower limit of the condition (1), the curvature becomes toolarge, it becomes difficult to correct the aberration, and the positionof the prism approaches near the face and the wear is impaired.

The condition (2) defines a difference between the curvature of thethird surface in the x axis direction and the curvature in the y axisdirection. When the calculated values exceed the upper limit of thecondition (2), the astigmatism generated in the third surface becomestoo large, and it becomes difficult to correct the aberration.

The condition (3) defines a difference between the curvature of thefirst surface and the curvature of the third surface with respect to thex axis direction and the y axis direction, and affects an influence tothe visibility of the prism with respect to the external light. When thethickness of the prism is T and the refractive index is N, thevisibility Dx in the x axis direction on the optical axis of the prismand the visibility Dy in the y axis direction are influenced accordingto:

Dx=2000(N−1)(A1_(2,0) −A3_(2,0)+(2T(N−1)/N)×A1_(2,0) ×A3_(2,0))

Dy=2000(N−1)(A1_(0,2) −A3_(0,2)+(2T(N−1)/N)×A1_(0,2) ×A3_(0,2)).

In general, if a tolerance of distant visibility exceeds ±1D, theobserver feels uncomfortable, and accordingly, it is preferable to keepthe visibility of the prism as ±1D. However, as the design, there isalso a case where the visibility on the optical axis is set as be in therange of ±2D due to the balance between the visibility of the outerperiphery portion and the aberration. As expressed in the expression,since the visibility on the optical axis is related to the thickness ofthe prism and the refractive index also, the visibility is notdetermined by only the values of the aspherical coefficients. However,if the coefficients are in the range of satisfying the condition (3), itis possible to keep the visibility on the optical axis to be in therange of ±2D.

By making the shape of the first surface and the third surface satisfythe above-described conditions (1) to (3), the aberration correction forboth of the external light and the image light can be performed, and itis possible to provide an excellent image quality.

In another specific aspect of the invention, in the virtual imagedisplay device, a half mirror is formed on the second surface and theimage light is presented to the observer. In that case, a lighttransmitting member may be integrally disposed outside of the secondsurface and the visibility with respect to the external light issubstantially zero, and then the external light and the image light maybe presented to the observer overlapped. In this case, it is possible toreduce the defocus or distortion of the external light when observingthe external light over the second surface.

In another specific aspect of the invention, in the virtual imagedisplay device, the light transmitting member includes a firsttransmitting surface and a second transmitting surface in the observerside, and a third transmitting surface in the external side, the secondsurface of the prism and the second transmitting surface of the lighttransmitting member have substantially the same curved surface shapes,and the second surface and the second transmitting surface areintegrated. In this case, two surfaces can be integrated by bonding, andthus, it is possible to form continuous surfaces on the first surfaceside and the third surface side respectively.

In another specific aspect of the invention, the virtual image displaydevice further includes a projection lens that causes the image lightfrom the image element to be incident on the prism. At least a part ofthe prism and the projection lens may configure a relay optical systemin which an intermediate image is formed.

In another specific aspect of the invention, in the virtual imagedisplay device, the projection lens is formed of an axisymmetric lensand includes at least one or more aspherical surfaces.

In another specific aspect of the invention, in the virtual imagedisplay device, the prism is disposed to face the projection lens andincludes a fourth surface that causes the image light emitted from theprojection lens to be incident and guide to the third surface.

In another specific aspect of the invention, in the virtual imagedisplay device, the prism includes a first prism portion of the lightemitting side including the first surface, the second surface, and thethird surface, and a second prism portion of the light incident side,and the first prism portion and the second prism portion are integrated.In this case, the intermediate image can be formed in the prism in whichthe first prism portion and the second prism portion are integrallyformed, at this time, by performing the total reflection at the firstsurface and the third surface of the first prism portion in multipletimes, it is possible to reduce the occurrence of discontinuity of thesurface related to the light guide of the image light.

In another specific aspect of the invention, in the virtual imagedisplay device, the second prism portion includes at least one or moreoptical surface and the intermediate image is formed by the imageelement, the projection lens, and at least a part of the prism where thesecond prism portion is included. In this case, at least the opticalsurface of the second prism portion contributes to forming of theintermediate image as a part of the relay optical system.

In another specific aspect of the invention, in the virtual imagedisplay device, the image element is an image display element that emitsan image light from the display position, and the projection lens and atleast a part of the prism where the second prism portion is includedcause the image light emitted from the display position of the imagedisplay element to form an image in the prism to form the intermediateimage, as the relay optical system. In this case, by the projection lensor the like functioning as the relay optical system, the image lightemitted from each point on the display position of the image displayelement is re-imaged in the prism and the intermediate image can beformed.

In another specific aspect of the invention, in the virtual imagedisplay device, in the third surface, the first prism portion includes afirst region where the image light passed through the second prismportion is totally reflected in a first time, and a second region wherethe image light is totally reflected in a second time, and theintermediate image is formed by the projection lens, the second prismportion, and a part where the first region of the first prism portion isincluded. In this case, in addition to the projection lens and thesecond prism portion, a part of the first prism portion contributes toforming of the intermediate image as a part of the relay optical system.

In another specific aspect of the invention, in the virtual imagedisplay device, the first prism portion and the second prism portioncause the intermediate image to be formed in front and back of the firstregion among the third surface, in a state of folded back. In this case,it is possible to reduce the size of the entire device.

In another specific aspect of the invention, in the virtual imagedisplay device, the second prism portion includes a fourth surface thatis disposed to face the projection lens and causes the image lightemitted from the projection lens to be incident on and guides to thethird surface, and a fifth surface that interposes the fourth surfacetogether with the third surface of the first prism portion. In this way,by the third and fifth surfaces, the fourth surface can ensure asufficiently large size and the direction according to the incidentimage light.

In another specific aspect of the invention, in the virtual imagedisplay device, the gap between the first surface and the third surfaceis equal to or more than 5 mm and is equal to or less than 15 mm. Inthis case, by making the gap equal to or more than 5 mm, it is possibleto sufficiently increase the size of the first prism to cover the frontof the eyes. By making the gap equal to or less than 15 mm, it ispossible to suppress the increase of the weight.

In another specific aspect of the invention, in the virtual imagedisplay device, an inclination angle of the second surface with respectto the first surface is equal to or more than 20° and is equal to orless than 40°. In this case, by making the inclination angle be in theabove-described range, it becomes easy to guide the image light to theeyes in the appropriate number of reflections and reflection angles.

In another specific aspect of the invention, in the virtual imagedisplay device, when the device is mounted on, the optical system thatincludes the prism covers a part of the front of the observer's eyes,and a part where the front of the eyes is not covered exists.

In another specific aspect of the invention, in the virtual imagedisplay device, the image element includes a signal light formingsection that emits a signal light which is modulated based on the image,and a scanning optical system that emits the signal light as a scanninglight by scanning the signal light incident on from the signal lightforming section.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a perspective view illustrating an external appearance of avirtual image display device in the embodiment.

FIG. 2 is a perspective view illustrating a structure of a main body ofthe virtual image display device.

FIG. 3A is a cross-sectional view of the plan view of the main bodyportion of a first display device that forms the virtual image displaydevice, and FIG. 3B is a front view of the main body portion.

FIG. 4 is a cross-sectional view illustrating a light surface and alight path in a prism in the first display device.

FIG. 5 is a diagram illustrating an optical system in ApplicationExample 1.

FIGS. 6A to 6F are diagrams illustrating aberrations of an opticalsystem in Application Example 1.

FIGS. 7A to 7F are diagrams illustrating aberrations of the opticalsystem in Application Example 1.

FIG. 8 is a diagram illustrating an optical system in ApplicationExample 2.

FIGS. 9A to 9F are diagrams illustrating aberrations of an opticalsystem in Application Example 2.

FIGS. 10A to 10F are diagrams illustrating aberrations of the opticalsystem in Application Example 2.

FIG. 11 is a diagram illustrating an optical system in ApplicationExample 3.

FIGS. 12A to 12F are diagrams illustrating aberrations of an opticalsystem in Application Example 3.

FIGS. 13A to 13F are diagrams illustrating aberrations of the opticalsystem in Application Example 3.

FIG. 14 is a diagram illustrating an optical system in ApplicationExample 4.

FIGS. 15A to 15F are diagrams illustrating aberrations of an opticalsystem in Application Example 4.

FIGS. 16A to 16F are diagrams illustrating aberrations of the opticalsystem in Application Example 4.

FIG. 17 is a diagram illustrating a virtual image display device inmodification example.

FIG. 18A is a perspective view illustrating a light guide device andanother example of a virtual image display device using the light guidedevice, and FIG. 18B is a front view thereof.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereafter, a virtual display device in an embodiment of the inventionwill be described in detail with reference to the drawings.

A. An External Appearance of a Virtual Image Display Device

A virtual image display device 100 illustrated in FIG. 1 is a head mountdisplay having an external appearance that is similar to glasses. Thevirtual image display device 100 can cause an observer who wears thevirtual image display device 100 to recognize an image light whichcorresponds to the virtual image, and can cause the observer torecognize or observe the outside image in a see-through. The virtualimage display device 100 includes a fluoroscopic member 101 that coversthe front of the observer's eyes, a frame 102 that supports thefluoroscopic member 101, and a first and second built-in device sections105 a and 105 b that are added to a portion from a cover section on bothof the right and left ends of the frame 102 to a vine portion (temple)on the rearward. Here, the fluoroscopic member 101 is an optical member(a transparent eye cover) curved with thickness that covers the front ofthe observer's eyes, and is divided into two portions: a first opticalportion 103 a and a second optical portion 103 b. A first display device100A in which the first optical portion 103 a and the first built-indevice section 105 a illustrated in the left side of the drawing arecombined is a portion to form a virtual image for the right eye, andalso functions as a virtual image display unit alone. In addition, asecond display device 100B in which the second optical portion 103 b andthe second built-in device section 105 b illustrated in the right sideof the drawing are combined is a portion to form a virtual image for theleft eye, and also functions as a virtual image display unit alone.

B. A Structure of the Display Device

As illustrated in FIG. 2, FIG. 3A, FIG. 3B, and the like, the firstdisplay device 100A includes a projection fluoroscope 70 and an imagedisplay device 80. Among them, the projection fluoroscope 70 includes aprism 10 that is a light guide member, a light transmitting member 50,and a projection lens 30 for image forming. The prism 10 and the lighttransmitting member 50 are integrated by bonding, and are firmly fixedon the lower side of the frame 61 such that the upper surface 10 e ofthe prism 10 and the lower surface 61 e of the frame 61 are in contactwith each other. The projection lens 30 is fixed to the end portion ofthe prism 10 via a lens barrel 62 in which the projection lens 30 isaccommodated. The prism 10 and the light transmitting member 50 of theprojection fluoroscope 70 correspond to the first optical portion 103 ain FIG. 1 and the projection lens 30 and the image display device 80 ofthe projection fluoroscope 70 correspond to the first built-in devicesection 105 a in FIG. 1. Moreover, since the second display device 100Bin FIG. 1 has the same structure as the first display device 100A, andonly the right and left direction is inverted, the detail description ofthe second display device 100B will not repeated.

The prism 10 of the projection fluoroscope 70 is an arc-shaped andcurved member so as to be along the face in a plan view, and can beconsidered by dividing the prism 10 into two portions: a first prismportion 11 on the center side near the nose and a second prism portion12 on the peripheral side separated from the nose. The first prismportion 11 is disposed on the light emitting side and includes a firstsurface S11, a second surface S12, a third surface S13 as side surfaceswhich has an optical function, and the second prism portion 12 isdisposed on the light-incident side and includes a fourth surface S14 asa side surface which has an optical function and a fifth surface S15 asa side surfaces which does not have an optical function. Among these,the second surface S12 is disposed between the first surface S11 and thethird surface S13. The fourth surface S14 is adjacent to the thirdsurface S13 and is disposed facing the projection lens 30 in a state ofbeing separated from the first surface S11 via the fifth surface S15. Inaddition, the prism 10 includes a first side surface 10 e and a secondside surface 10 f that are adjacent to the first to fifth surfaces S11to S15 and are facing each other.

In the prism 10, the first surface S11 is a free-form surface with anemission side light axis AXO which is parallel to the Z axis as a centeraxis or a reference axis, and the second surface S12 is a free-curvedsurface with a light axis AX1 which is included in the reference surfaceSR parallel to the XZ plane and inclined with respect to the Z axis as acenter axis or a reference axis, the third surface S13 is a free-curvedsurface with an emission side light axis AXO as a as a center axis or areference axis. The fourth surface S14 is a free-curved surface with anincident side optical axis AXI that is an optical axis which is includedin the reference surface SR parallel to the XZ plane and inclined withrespect to the Z axis as a center axis or a reference axis. Moreover,above-described first to fourth surfaces S11 to S14 extend tohorizontally (laterally) and across the reference surfaces SR throughwhich the light axis AX1 to AX5 pass in parallel to the XZ plane, andhave a symmetrical shape with respect to the longitudinal Y axisdirection (vertical).

The prism 10 is formed of a resin material which shows a high opticaltransparency in the visible range, and, for example, is molded byinjecting and setting the thermoplastic resin into a mold. The main bodyportion 10 s of the prism 10 has an integrated formation. However, it ispossible to consider dividing it into a first prism portion 11 and asecond prism portion 12. The first prism portion 11 enables an imagelight GL to be guided and emitted and enables an external light HL tofluoroscope. The second prism portion 12 enables the image light GL tobe incident and guided.

In the first prism portion 11, the first surface S11 functions as arefraction surface which causes the image light GL to be emitted outsidethe first prism portion 11 and functions as a total reflection surfacewhich causes the image light GL to be totally reflected on the innersurface side of the image light GL. The first surface S11 is disposed infront of the eyes EY and forms a concave surface shape with respect tothe observer. Moreover, in the first surface S11, the main body portion10 s may be coated with a hard coating layer in order to prevent thedamage to the surface and the deterioration of the video resolution. Thehard coating layer is formed by depositing a coating agent made from theresin or the like through dip treatment or spray coating on the lowersurface of the main body portion 10 s.

The second surface S12 has a half mirror layer 15. The half mirror layer15 is a reflection film (that is, a transflective film) having a lighttransparency. The half mirror layer (the transflective film) 15 isformed on the partial area PA, not on all the area of the second surfaceS12. That is, the half mirror layer 15 is formed on the partial area PAwhere the all of the area QA on which the second surface S12 extends isnarrowed mainly with respect to a vertical direction. In more detail,the partial area PA is disposed on the center side with respect to thevertical Y direction, and substantially throughout with respect to thedirection along the horizontal reference surface SR. The half mirrorlayer 15 is formed by film-forming of a metal reflective film or adielectric multilayer film on a partial area PA of the lower surface ofthe main body portion 10 s. The reflectance of the half mirror layer 15with respect to the image light GL is to be equal to or higher than 10%and equal to or lower than 50% in the assumed range of incident angle ofthe image light GL under the view point of making the observing of theexternal light HL by see-through easy. The reflectance of image light GLof the half mirror layer 15 in the specific application example is, forexample, set as 20%, and hence, the transmittance with respect to theimage light GL is set as 80%.

The third surface S13 functions as a total reflection surface thatcauses the image light GL to be totally reflected in the inner surfaceside. Moreover, the main body portion 10 s of the third surface S13 mayalso be covered by the hard coating layer in order to prevent the damageon the surface and the decrease of the resolution. The third surface S13is disposed in front of the eyes EY, and is formed as a concave surfaceshape with respect to the observer similar to the first surface S11.Therefore, when the third surface S13 causes the external light HL topass through the first surface S11 and the third surface S13 and seesthe external light HL, the visibility is substantially zero. Moreover,the total reflection of the image light GL at the third surface S13 canbe considered in two ways; a first region P1 where the image light GLpassed through the fourth surface S14 is totally reflected (the firsttotal reflection) and a second region P2 where the image light GL passedthrough the first surface S11 is totally reflected (the second totalreflection).

The fourth surface S14 functions as a refraction surface that causes theimage light GL to be incident on inside the second prism portion 12.Moreover, in the fourth surface S14, the main body portion 10 s may becoated with the hard coating layer in order to prevent the surfacedamage and to prevent the resolution of the image from being decreased.However, instead of the hard coating layer, or in addition to the hardcoat layer, an antireflection film 17 may be coated.

The fifth surface S15 is a surface that links the first surface S11 andthe fourth surface S14, but as described above, does not have an opticalfunction. For this reason, the surface precision is rough compared tothe other surfaces S11 to S14, for example, it may also be acceptablethat the surface be appropriately rough so as to prevent irregularreflection. The fifth surface S15 interposes the fourth surface S14 oncooperation with the third surface S13. However, in the fourth surfaceS14 is in a state that a sufficient range of area can be secured withrespect to the incident angle of the image light GL.

The light transmitting member 50 is integrally fixed to the prism 10.The light transmitting member 50 is a member (auxiliary prism) thatassists the fluoroscopic function of the prism 10, and has a firsttransmitting surface S51, second transmitting surface S52, and thirdtransmitting surface S53 as side surfaces having optical functions.Here, the second transmitting surface S52 is disposed between the firsttransmitting surface S51 and the third transmitting surface S53. Thefirst transmitting surface S51 is on the curved surface that is anextension of the first surface S11 of the prism 10, the secondtransmitting surface S52 is a curved surface that is adhered andintegrated with respect to the second surface S12 by the adhesion layerCC, and the third transmitting surface S53 is on the curved surface thatis an extension of the third surface S13 of the prism 10. Among them,the second transmitting surface S52 and the second surface S12 of theprism 10 are integrated by the adhesion, thus, have a shape ofsubstantially same curvature.

The light transmitting member (auxiliary prism) 50 is formed of a resinmaterial showing a high optical transmittance in the visible range andhaving a substantially same refractive index as the main portion 10 s ofthe prism 10. The light transmitting member 50 is, for example, form bymolding of the thermoplastic resin.

The projection lens 30 is held in the lens barrel 62, and the imagedisplay device 80 is fixed to one end of the lens barrel 62. The secondprism portion 12 of the prism 10 is connected to the lens barrel 62which holds the projection lens 30 and indirectly supports theprojection lens 30 and the image display device 80. The light incidentside of the prism 10 is covered with the shielding member 63 togetherwith the projection lens 30. The upper end part or the lower end part ofthe prism 10 is also covered with the shielding member 63. In thevicinity of the prism 10, an additional shielding section may beprovided in order to prevent the external light from being incident onthe prism 10. The shielding section can be, for example, formed of acoating of light-shielding or a light scattering layer.

The projection lens 30, for example, has three lenses 31, 32, and 33along the incident-side light axis AXI. Each lens 31, 32 and 33 is anaxially symmetric lens, and has at least one or more aspheric surfaces.The projection lens 30 causes the image light GL emitted from the imagedisplay device 80 to be incident on the prism 10 via the fourth surfaceS14 of the prism 10 in order to re-image. That is, the projection lens30 is a relay optical system in order for the image light or video lightemitted from each point on the image plane (display position) OI of theimage display element 82 to be re-imaged in the prism 10. Moreover, apart of surface of the light-incident surface side among each of thesurfaces of the prism 10 functions as a part of the relay optical systemby cooperation with the projection lens 30.

The image display device 80 includes an illumination device 81 thatemits a two dimensional illumination light SL, a image display element82 that is a transmission type spatial light modulation device, and adrive control section 84 that controls the operations of theillumination device 81 and the image display element 82.

The illumination device 81 of the image display device 80 includes alight source 81 a that generates a light which includes three colors ofred, green, and blue, and a back-light light guide portion 81 b thatdiffuses the light emitted from the light source 81 a and makes a lightbeam having a rectangular cross section. The image display element 82is, for example, an image element formed on the liquid crystal displaydevice, and spatially modulates the illumination light SL emitted fromthe illumination device 81 and forms the video light which is to bedisplayed such as moving images. The drive control section 84 includes alight source drive circuit 84 a and a liquid crystal drive circuit 84 b.The light source drive circuit 84 a supplies power to the light source81 a of the illumination device 81 and causes the stable illuminationlight SL to be emitted. The liquid crystal drive circuit 84 b forms acolor image light which is a base of a moving image or a still image asa transmittance pattern by outputting an image signal or a drive signalwith respect to the image display element (image element) 82. Moreover,it is possible to give an image processing function to the liquidcrystal drive circuit 84 b, and it is also possible to give the imageprocessing function to an external control circuit.

C. Light Path of the Image Light

Hereafter, the light path of the image light GL in the virtual imagedisplay device 100 will be described.

The image light GL emitted from the image display element (imageelement) 82 is converged by the projection lens 30, and is incident onthe fourth surface S14 that is provided on the second prism portion 12of the prism 10 and has a comparatively strong positive refractivepower.

The image light GL passing through the fourth surface S14 of the prism10 continues to proceed with being converged, and is totally reflectedat the third surface S13 (more specifically, at the first region P1)(the first total reflection at the third surface S13) that has acomparatively weak positive refractive power when passing through thefirst prism portion 11, and then, is totally reflected at the firstsurface S11 that has a comparatively weak negative refractive power (thefirst total reflection at the first surface S11).

In the first prism portion 11, the image light GL reflected at the firstsurface S11 is incident again on the third surface S13 (morespecifically, the second region P2) and is totally reflected (the secondtotal reflection at the third surface S13), and incident on the firstsurface S11 and is totally reflected (the second total reflection at thefirst surface S11). Moreover, the image light GL forms an intermediateimage in the prism 10 before and after passing through the third surfaceS13 in the first time. In other words, the intermediate image of theimage light GL emitted from the image display element 82 is formed bythe projection lens 30, the second prism portion 12, and a part in whichthe first region P1 of the first prism portion 11 is included. The imageplane II of the intermediate image corresponds to the image plane(display position) OI of the image display element 82, but has a shapebeing folded back by the third surface S13.

The image light GL which is totally reflected at the first surface S11second time is incident on the second surface S12, but the image lightGL particularly incident on the half mirror layer 15 partially transmitsthe half mirror layer 15 and partially reflected and again incident onthe first surface S11 and passes through. Moreover, the half mirrorlayer 15 acts as a surface having a comparatively strong positiverefractive power with respect to the image light GL reflected here. Inaddition, the first surface S11 acts as a surface having a negativerefractive power with respect to the image light GL passing through.

The image light GL passing through the first surface S11 is incident onthe pupil of the observer's eye EY as a substantially parallel lightbeam. That is, the observer observes the image formed on the imagedisplay element 82 by the image light GL as a virtual image.

In contrast, among the external light HL, the light incident on the +Xside compared to the second surface S12 of the prism 10 passes throughthe third surface S13 and the first surface S11 of the first prismportion 11, and at this time, the positive and negative refraction powerare cancelled out and the aberration is corrected. That is, the observerobserves the outside image with less distortion over the prism 10.Similarly, among the external light HL, when the light incident on the−X side compared to the second surface S12 of the prism 10, that is,incident on the light transmitting member 50 passes through the thirdtransmitting surface S53 and the first transmitting surface S51 providedhere, the positive and negative refraction power are cancelled out andthe aberration is corrected. That is, the observer observes the outsideimage with less distortion over the prism 10 and the light transmittingmember 50. Furthermore, among the external light HL, when the lightincident on the light transmitting member 50 which corresponds to thesecond surface S12 of the prism 10 passes through the third transmittingsurface S53 and the first surface S11, the positive and negativerefraction power are cancelled out and the aberration is corrected. Thatis, the observer observes the outside image with less distortion overthe light transmitting member 50. Moreover, the second surface S12 ofthe prism 10 and the second transmitting surface S52 of the lighttransmitting member 50 has substantially the same curved surface shapeand refractive index respectively, a gap therebetween is filled with anadhesive layer CC of the substantially same refractive index. That is,the second surface S12 of the prism 10 and the second transmittingsurface S52 of the light transmitting member 50 does not act as arefractive surface with respective the external light HL.

However, since the external light HL incident on the half mirror layer15 is partly transmitted and partly reflected at the half mirror layer15, the external light HL from the direction corresponding to the halfmirror layer 15 becomes weak based on the transmittance of the halfmirror layer 15. In contrast, from the direction corresponding to thehalf mirror layer 15, the image light GL is incident. Accordingly, theobserver observes the outside image in the direction of half mirrorlayer 15 together with the image formed on the image display element 82.

Among the image light GL propagated in the prism 10 and incident on thesecond surface S12, the light which is not reflected at the half mirrorlayer 15 is incident on the light transmitting member 50, but isprevented from returning to the prism 10 due to a not illustratedanti-reflection section provided on the light transmitting member 50.That is, the image light GL passed through the second surface S12 isprevented from being returned to the light path and from becoming astrayed light. In addition, the external light HL incident from thelight transmitting member 50 and reflected at the half mirror layer 15is returned to the light transmitting member 50, but is prevented frombeing emitted to the prism 10 due to the above-described and notillustrated anti-reflection section provided on the light transmittingmember 50. That is, the external light HL reflected at the half mirrorlayer 15 is prevented from being returned to the light path and frombecoming a strayed light.

D. Method of Defining the Optical Surface and Light Path

FIG. 4 is a diagram illustrating light axes AX1 to AX5 and a localcoordinate in the prism 10. In the description hereafter, consideringthe evaluation in the optical system or the convenience of theexpression, the optical plane and the light path are defined regarding areverse direction from the observer's eyes EY toward the image displayelement 82 of the image display device 80. In the optical system inactual, the light generated from the image display element 82sequentially passes through the projection lens 30 and the prism 10, andarrives at the eyes EY. However, in that situation, it is difficult toevaluate the optical system. For this reason, since the optical systemis evaluated and designed as the light from the light source at infinitypasses through a diaphragm at the position of the eyes EY and is inputto the prism 10, and then passes through the projection lens 30 to forman image on the image display element 82, data of the optical systemdescribed in detail hereafter is also expressed in such an order.Moreover, regarding the light transmitting member 50 which is used as anintegrated unit by being joined to the prism 10, the shape thereof isobtained by extending the shape of the prism 10, and thus, thedescription is omitted.

In the prism 10 illustrated, the light axis on the first surface S11coincides with the emitting side light axis AXO, and the localcoordinate (x, y, z) of the first surface S11 is in a translationrelations with the global coordinate (X, Y, Z) and has an origin on thefirst surface S11. That is, z direction in the local coordinate is onthe emitting side light axis AXO which is an advancing direction(reverse direction of the light), and y direction in the localcoordinate is parallel to the Y direction in the global coordinate. Ineach surface hereafter, the y direction in the local coordinate isparallel to the Y direction in the global coordinate.

The light axis on the second surface S12 is appropriately inclined withrespect to the emitting side light axis AXO, the local coordinate of thesecond surface S12 is appropriately rotated around the Y axis withrespect to the global coordinate and keeps the translation relation, andhas an origin on the second surface S12. The z direction in the localcoordinate on the second surface S12 is an intermediate directionbetween the emitting side light axis AXO and the light axis AX1 of thecenter of the light beam toward the first surface S11 from the secondsurface S12.

The light axis on the third surface S13 coincides with the emitting sidelight axis AXO, the local coordinate of the third surface S13 is in atranslation relations with the global coordinate and has an origin onthe extended surface of the third surface S13, that is, on the thirdtransmitting surface S53.

As described above, the intermediate direction between the light axisAX1 of the center of the light beam toward the first surface S11 fromthe second surface S12 and the light axis AX2 of the center of the lightbeam toward the third surface S13 from the first surface S11 coincideswith a normal direction of the first surface S11 in the center of thelight beam (an intersection of light axes AX1 and AX2) on the firstsurface S11. In addition, the intermediate direction between the lightaxis AX2 of the center of the light beam toward the third surface S13from the first surface S11 and the light axis AX3 of the center of thelight beam toward the first surface S11 from the third surface S13coincides with a normal direction of the third surface S13 in the centerof the light beam (an intersection of light axes AX2 and AX3) on thethird surface S13.

In the light path toward the first surface S11 from the third surfaceS13 again, the local coordinate corresponds to the advancing direction(a reverse direction of the light). That is, the z direction in thelocal coordinate from the third surface S13 to the first surface S11coincides with the light axis AX3 of the center of the light beam, andthe y direction in the local coordinate is parallel to the Y directionin the global coordinate. Moreover, light axis AX4 of the center of thelight beam is extended by folding the light axis AX3 of the center ofthe light beam at the first surface S11, and the light axis AX5 of thecenter of the light beam is extended by folding again the light axis AX4of the center of the light beam at the third surface S13.

The optical axis of the fourth surface S14 coincides with the opticalaxis AX5 in which the optical axis AX3 is folded back at the firstsurface S11 and the third surface S13, and coincides with theincident-side optical axis AXI which is extended from the image displaydevice 80. In the optical axis of the fourth surface S14, the localcoordinate on the fourth surface S14 is in the translation relation andhas an origin on the fourth surface S14.

As described above, as a result of tracing in the reverse direction ofthe light beam, for the traveling direction of the light beam, bytracing above-described each light axes AX1 to AX5 from theincident-side light axis AXI which is the center of the light beam ofthe image light emitted from the center of the image plane OI, andleading to the emitting side light axis AXO in the reverse order, thelight arrives at the observer's eyes EY.

E. Preferable Characteristics of Optical Surface

The shape of the first surface S11 of the prism 10 is expressed as

z=Σ{A1_(m,n)·(x ^(m) ·y ^(n))}  (4)

using the local coordinate (x, y, z) of the first surface S11. Here, A1_(m,n) is a coefficient of the m·n_(th) term of expanded polynomial. mand n are integers equal to or larger than zero.

The shape of the second surface S12 of the prism 10 is expressed as

z=Σ{A2_(m,n)·(x ^(m) ·y ^(n))}  (5)

using the local coordinate (x, y, z) of the second surface S12. Here, A2_(m,n) is expressed by a coefficient of the m·n_(th) term of expandedpolynomial.

The shape of the third surface S13 of the prism 10 is expressed as

z=Σ{A3_(m,n)·(x ^(m) ·y ^(n))}  (6)

using the local coordinate (x, y, z) of the third surface S13. Here, A3_(m,n) expressed by a coefficient of the m·n_(th) term of expandedpolynomial.

The first to third surfaces S11 to S13 of the prism 10 in the embodimentsatisfies three conditions as follows.

−5×10⁻² <A1_(2,0) +A1_(0,2)<−1×10⁻³ and −5×10⁻² <A3_(2,0)+A3_(0,2)<−1×10⁻³  (1)

|A3_(2,0) −A3_(0,2)|<5×10⁻²  (2)

|A1_(2,0) −A3_(2,0)|<5×10⁻³ and |A1_(0,2) −A3_(0,2)|<5×10⁻³  (3)

By setting the shape of the first to third surfaces S11 to S13 so as tosatisfy the above-described three conditions, the aberration correctionof both of the external light HL and the image light GL is successfullyperformed, and it is possible to provide excellent image quality.

The gap between the first surfaces S11 and the third surface S13 of theprism 10 is equal to or larger than 5 mm and equal to or smaller than 15mm. In addition, the inclination angle of the second surface S12 withrespect to the first surface S11 is equal to or larger than 20° andequal to or smaller than 40°.

In the virtual image display device 100 according to the embodiment, anintermediate image is formed in the prism 10 by the projection lens 30and the like, and the image light GL that is totally reflected at threeor more surfaces: the third surface S13 firstly, the first surface S11firstly, the third surface S13 secondly, the first surface S11 secondly,and the second surface S12 in this order, and then transmits through thefirst surface S11 and then arrives at the observer's eye EY.Accordingly, the prism 10 can be made thin in thickness and can bereduced in the size and weight of the entire optical system, and then itis possible to realize a display of high performance and brightness witha wide angle of view. In addition, the external light HL, for example,can be observed after passing through the first surface S11 and thethird surface S13, and the visibility at that time becomes substantiallyzero. Therefore, it is possible to reduce the defocus or distortion ofthe external light HL when observing the external light HL insee-through. In addition, the shape the prism 10 is along the face ofthe observer, the center of gravity can be close to the face, and thedesign can also be made excellent. In addition, in the embodimentdescribed above, since the prism 10 is configured that the totalreflection is performed multiple times in the first surface S11 and thethird surface respectively, the generation of discontinuous part in thesurface involved in the light guiding among the surfaces of the prism 10is reduced. As a result, the occurrence of such a situation that theghost caused by the unintended reflection or the like at thediscontinuous part of the prism can be avoided.

APPLICATION EXAMPLES

Hereafter, an application example of a projection fluoroscope that isincorporated in a virtual image display device according to theinvention will be described. The symbols used in each applicationexample are summarized as follows.

SPH: pupilFFSk: free-curved surface (k in the prism=surface number)ASPk: Axisymmetric aspherical surface (k in the projection opticalsystem=surface number)SPH: spherical surface or plane (protective glass surface)R: radius of curvatureT: axial surface distanceNd: refractive index with respect to the line d of the optical materialVd: Abbe number with respect to the line d of the optical materialTLY: inclination angle)(° of the optical axis in a cross-section surface(XZ section) in a specific surface (There is a case where the TLY variesat the front and back of the specific surface)DCX: amount of shift of the optical axis in an X direction in across-section surface (XZ cross section) in a specific surface

Application Example 1

Data for optical surface that configures the prism and the projectionlens of the projection fluoroscope in the application example 1 is shownin Table 1 below. Moreover, for example, FFS 1 means the first surfaceS11, FFS2 means the second surface S12, and FFS3 means the third surfaceS13. In addition, ASP1 means the emitting surface of the first lens ofthe projection lenses, and ASP2 means the incident surface of the firstlens.

TABLE 1 No Type R T Nd Vd 1 SPH ∞ 20.00 2 FFS1 −448.679 5.50 1.525 55.953 FFS2 −96.942 −5.50 1.525 55.95 4 FFS1 −448.679 10.00 1.525 55.95 5FFS3 −467.374 −10.00 1.525 55.95 6 FFS1 −448.679 10.00 1.525 55.95 7FFS3 −467.374 −31.00 1.525 55.95 8 FFS4 11.319 −3.00 9 ASP1 −13.776−6.00 1.525 55.95 10 ASP2 −12.233 −2.10 11 ASP3 −33.306 −1.50 1.58529.90 12 ASP4 −117.670 −9.12 13 ASP5 −7.154 −6.50 1.525 55.95 14 ASP69.735 −6.86 15 SPH ∞ −1.44 1.458 67.82 16 image surface

Regarding the optical surface of the prism that configures theapplication example 1, the inclination angle (tilt) TLY of the opticalaxis in the cross-section in the cross-section surface thereof, and theamount of shift (de-centered) DCX of the optical axis is shown in Table2 described below.

TABLE 2 No Type TLY (front) DCX (back) TLY (back) 2 FFS1 0 0 0 3 FFS2−28 0 28 4 FFS1 0 0 0 5 FFS3 0 0 0 6 FFS1 0 0 0 7 FFS3 0 48.99 −32.5 8FFS4 0 0 0

Regarding each optical surface of the prism that configures theapplication example 1, the coefficient Ak_(m,n) expressed by theexpanded polynomial of the free-curved surface is shown in Table 3below. Here, in Table 3, the symbols m and n mean the variables or theorders in the coefficient Ak_(m,n). In addition, the symbol FFSk (k=1 to4) means the k_(th) surface among the first to fourth surfaces S11 toS14 which are the free-curved surfaces. Moreover, the coefficientAk_(m,n) means the coefficient of each term x^(m)·y^(n) which configuresthe polynomial that expresses the subjected k_(th) surface.

TABLE 3 m n FFS1 FFS2 FFS3 FFS4 2 0 −1.114E−03 −5.158E−03 −1.070E−03  4.417E−02 0 2 −1.673E−02 −9.940E−03 −1.505E−02   5.763E−02 3 0−1.010E−04 −9.079E−05 −6.852E−05   1.374E−04 1 2   6.213E−04   2.871E−04  4.214E−04   3.659E−04 4 0   4.318E−07   7.789E−10   2.219E−07−2.346E−04 2 2 −1.409E−05 −2.537E−05 −7.241E−06 −4.796E−04 0 4−3.174E−05 −1.405E−05 −1.631E−05 −2.464E−04 5 0   1.821E−08 −4.769E−08  7.088E−09 −1.014E−06 3 2   3.813E−07   9.033E−07   1.484E−07  2.275E−07 1 4 −5.398E−06 −2.474E−06 −2.101E−06 −2.227E−06 6 0−1.606E−10   1.278E−08 −4.737E−11 −7.275E−07 4 2 −3.691E−09   2.334E−08−1.088E−09 −1.525E−06 2 4   9.187E−08   1.909E−07   2.709E−08 −2.201E−070 6   1.402E−06   2.671E−07   4.134E−07 −7.202E−07

In above-described Table 3 and Tables described below, after E of thenemeric value means the exponent of the decimal numbers, for example,“−1.114E−03” means −1.114×10⁻⁰³.

The coefficient of the aspherical surface of the optical surface thatconfigures the projection lens of the projection fluoroscope in theapplication example 1 is shown in Table 4 below.

TABLE 4 ASP1 ASP2 ASP3 ASP4 ASP5 ASP6 K −1 −1 −1 −1 −1 −1 B4 −3.858E−05−2.098E−03 −8.922E−03 −8.952E−03 1.149E−03 −3.262E−04  B6 −1.414E−05 8.369E−05  6.655E−04  9.272E−04 −2.564E−05  8.974E−06 B8  7.806E−08−6.017E−07 −1.567E−05 −4.019E−05 6.815E−07 1.260E−07

In Table 4 described above, the symbols K and Bi indicate thecoefficients for specifying an aspherical surface out of the aspericalsurfaces APS1 to APS6 which are the lens surfaces of three lenses 31 to33 that configure the projection lens 30. The aspherical surface isspecified by a polynomial (equation of aspherical surface) below.

$z = {\frac{\left( {1/R} \right) \times h^{2}}{1 + \sqrt{1 - {\left( {K + 1} \right) \times \left( {1/R} \right)^{2} \times h^{2}}}} + {B_{4}h^{4}} + {B_{6}h^{6}} + {B_{6}h^{8}} + \ldots}$

Here, R is a radius of the curvature of each surface, h is a height fromthe optical axis, K is a conical coefficient of the subjected lenssurface, and Bi (I=4, 6, 8, . . . ) is a high order asphericalcoefficient of the subjected lens surface.

FIG. 5 is a cross-sectional diagram of the projection fluoroscope 70 inthe application example 1. However, not only the light beam on thereference surface SR but also the light beam away from the referencesurface SR in the Y direction is illustrated. The prism 10 of theprojection fluoroscope 70 includes the first surface S11 that has arelatively weak negative refractive power, the second surface S12 thathas a relatively strong positive refractive power, the third surface S13that has a relatively weak positive refractive power, the fourth surfaceS14 that has a relatively strong positive refractive power. Theprojection lens 30 includes a first lens 31 that has a positiverefractive power, a second lens 32 that has a negative refractive power,a third lens 33 that has a positive refractive power, and all of thesurfaces thereof is aspherical surfaces as described above. The detailspecification of the optical system in the application example 1 will bedescribed. A horizontal angle of view is 20.1°, a vertical angle of viewis 11.4°, a size of the display area of the image display element is9.22× 5.18 mm, a diameter of a pupil is 5 mm, and a focal length isapproximately 26 mm.

FIGS. 6A to 6F and FIGS. 7A to 7F illustrate the aberrations in theapplication example 1. In each aberration diagram, a horizontal axisrepresents the position in the pupil, and a vertical axis represents anamount of aberration in micron unit. Specifically, FIGS. 6A and 6Bindicates the aberration in Y and X directions in the direction of 10°in the X direction and 5.7° in the Y direction, and FIGS. 6C and 6Dindicate the aberration in Y and X directions in the direction of 0.0°in the X direction and 5.7° in the Y direction, and FIGS. 6E and 6Findicate the aberration in Y and X directions in the direction of −10°in the X direction and 5.7° in the Y direction. FIGS. 7A and 7B indicatethe aberration in Y and X directions in the direction of 10° in the Xdirection and 0.0° in the Y direction, FIGS. 7C and 7D indicate theaberration in Y and X directions in the direction of 0.0° in the Xdirection and 0.0° in the Y direction, and FIGS. 7E and 7F indicate theaberration in Y and X directions in the direction of −10° in the Xdirection and 0.0° in the Y direction. Moreover, the illustrated amountof aberration is the amount of aberration in the image plane of theimage display element in a case where the light reversely travels forthe sake of convenience in explanation.

Application Example 2

Data for optical surface that configures the prism and the projectionlens of the projection fluoroscope in the application example 2 is shownin Table 5 below.

TABLE 5 No Type R T Nd Vd 1 SPH ∞ 20.00 2 FFS1 −78.850 5.50 1.525 55.953 FFS2 −49.956 −5.50 1.525 55.95 4 FFS1 −78.850 10.00 1.525 55.95 5 FFS3−82.135 −10.00 1.525 55.95 6 FFS1 −78.850 10.00 1.525 55.95 7 FFS3−82.135 −32.00 1.525 55.95 8 FFS4 63.555 −3.00 9 ASP1 −5.163 −8.00 1.52555.95 10 ASP2 10.084 −0.50 11 ASP3 9.011 −1.00 1.585 29.90 12 ASP4−5.154 −7.02 13 ASP5 −11.265 −6.50 1.525 55.95 14 ASP6 6.033 −6.06 15SPH ∞ −1.44 1.458 67.82 16 image surface

Regarding the optical surface of the prism that configures theapplication example 2, the inclination angle (tilt) TLY of the opticalaxis in the cross-section surface thereof, and the amount of shift(de-centered) DCX of the optical axis is shown in Table 6 describedbelow.

TABLE 6 No Type TLY (front) DCX (back) TLY (back) 2 FFS1 0 0 0 3 FFS2−28 0 28 4 FFS1 0 0 0 5 FFS3 0 0 0 6 FFS1 0 0 0 7 FFS3 0 46.23 −9.4 8FFS4 0 0 0

Regarding each optical surface of the prism that configures theapplication example 2, the coefficient expressed by the expandedpolynomial of the free-curved surface is shown in Table 7 below. Here,in Table 7, the symbols m and n mean the variables or the orders in thecoefficient Ak_(m,n). In addition, the symbol FFSk (k=1 to 4) means thek_(th) surface among the first to fourth surfaces S11 to S14 which arethe free-curved surfaces.

TABLE 7 m n FFS1 FFS2 FFS3 FFS4 2 0 −6.341E−03 −1.001E−02 −6.088E−03  7.867E−03 0 2 −1.288E−02 −1.005E−02 −1.194E−02   2.674E−02 3 0−5.819E−05   1.216E−05 −3.947E−05   1.707E−04 1 2   1.612E−04  7.366E−05   1.093E−04   5.790E−04 4 0   1.024E−06 −2.011E−06  5.264E−07   2.142E−04 2 2   2.067E−06 −3.685E−06   1.062E−06  4.355E−04 0 4 −2.040E−05 −8.151E−06 −1.048E−05   1.888E−04 5 0−6.146E−08   2.096E−08 −2.393E−08 −1.084E−06 3 2   5.673E−08   1.093E−07  2.208E−08   9.286E−08 1 4 −1.833E−06 −5.627E−07 −7.135E−07 −1.769E−076 0   9.259E−10 −1.693E−10   2.730E−10 −1.241E−06 4 2   9.914E−10−2.381E−08   2.924E−10 −3.355E−06 2 4   1.259E−08   1.133E−08  3.713E−09 −2.861E−06 0 6   2.496E−07   6.872E−09   7.360E−08−1.059E−06

The coefficient of the aspherical surface of the optical surface thatconfigures the projection lens of the projection fluoroscope in theapplication example 2 is shown in Table 8 below.

TABLE 8 ASP1 ASP2 ASP3 ASP4 ASP5 ASP6 K −1 −1 −1 −1 −1 −1 B4 −2.238E−04−1.670E−03 −6.772E−03 −9.690E−03 −5.465E−05  −5.000E−04 B6 −6.846E−06 4.973E−05  3.135E−04 −9.444E−04 5.815E−07 −5.024E−06 B8  1.908E−07−6.137E−07 −6.129E−06  1.511E−04 8.128E−09  1.002E−07

In Table 8 described above, the symbols K and Bi indicate thecoefficients for specifying an aspherical surface out of the aspericalsurfaces APS1 to APS6 which are the lens surfaces of three lenses 31 to33 that configure the projection lens 30.

FIG. 8 is a cross-sectional diagram of the projection fluoroscope 70 inthe application example 2. The prism 10 of the projection fluoroscope 70includes the first surface S11 that has a relatively weak negativerefractive power, the second surface S12 that has a relatively strongpositive refractive power, the third surface S13 that has a relativelyweak positive refractive power, and the fourth surface S14 that has arelatively strong positive refractive power. The projection lens 30includes a first lens 31 that has a positive refractive power, a secondlens 32 that has a negative refractive power, a third lens 33 that has apositive refractive power. The detail specification of the opticalsystem in the application example 2 will be described. A horizontalangle of view is 20.1°, a vertical angle of view is 11.4°, a size of thedisplay area of the image display element is 9.22×5.18 mm, a diameter ofa pupil is 5 mm, and a focal length is approximately 26 mm.

FIGS. 9A to 9F and FIGS. 10A to 10F illustrate the aberrations in theapplication example 2. Specifically, FIGS. 9A and 9B indicates theaberration in Y and X directions in the direction of 10° in the Xdirection and 5.7° in the Y direction, and FIGS. 9C and 9D indicate theaberration in Y and X directions in the direction of 0.0° in the Xdirection and 5.7° in the Y direction, and FIGS. 9E and 9F indicate theaberration in Y and X directions in the direction of −10° in the Xdirection and 5.7° in the Y direction. FIGS. 10A and 10B indicate theaberration in Y and X directions in the direction of 10° in the Xdirection and 0.0° in the Y direction, FIGS. 10C and 10D indicate theaberration in Y and X directions in the direction of 0.0° in the Xdirection and 0.0° in the Y direction, and FIGS. 10E and 10F indicatethe aberration in Y and X directions in the direction of −10° in the Xdirection and 0.0° in the Y direction.

Application Example 3

Data for optical surface that configures the prism and the projectionlens of the projection fluoroscope in the application example 3 is shownin Table 9 below.

TABLE 9 No Type R T Nd Vd 1 SPH ∞ 20.00 2 FFS1 −126.777 5.50 1.525 55.953 FFS2 −74.042 −5.50 1.525 55.95 4 FFS1 −126.777 10.00 1.525 55.95 5FFS3 −132.059 −10.00 1.525 55.95 6 FFS1 −126.777 10.00 1.525 55.95 7FFS3 −132.059 −30.00 1.525 55.95 8 FFS4 11.731 −3.00 9 ASP1 −12.215−6.00 1.525 55.95 10 ASP2 −7.954 −8.35 11 ASP3 −9.712 −6.50 1.525 55.9512 ASP4 5.998 −10.01 13 SPH ∞ −1.44 1.458 67.82 14 image surface

Regarding the optical surface of the prism that configures theapplication example 3, the inclination angle (tilt) TLY of the opticalaxis in the cross-section surface thereof, and the amount of shift(de-centered) DCX of the optical axis is shown in Table 10 describedbelow.

TABLE 10 No Type TLY (front) DCX (back) TLY (back) 2 FFS1 0 0 0 3 FFS2−28 0 28 4 FFS1 0 0 0 5 FFS3 0 0 0 6 FFS1 0 0 0 7 FFS3 0 46.82 −23.8 8FFS4 0 0 0

Regarding each optical surface of the prism that configures theapplication example 3, the coefficient expressed by the expandedpolynomial of the free-curved surface is shown in Table 11 below. Here,in Table 11, the symbols m and n mean the variables or the orders in thecoefficient Ak_(m,n). In addition, the symbol FFSk (k=1 to 4) means thek_(th) surface among the first to fourth surfaces S11 to S14 which arethe free-curved surfaces.

TABLE 11 m n FFS1 FFS2 FFS3 FFS4 2 0 −3.944E−03 −6.753E−03 −3.786E−03  4.262E−02 0 2 −1.423E−02 −9.039E−03 −1.299E−02   5.801E−02 3 0−2.504E−05 −2.202E−05 −1.698E−05   1.668E−04 1 2   5.556E−04   2.511E−04  3.768E−04   1.989E−04 4 0 −1.253E−06 −4.555E−06 −6.438E−07 −2.328E−042 2 −1.114E−05 −2.283E−05 −5.724E−06 −5.034E−04 0 4 −5.147E−05−2.479E−05 −2.645E−05 −2.842E−04 5 0   3.097E−08   1.293E−07   1.205E−08−7.239E−07 3 2   3.512E−07   5.492E−07   1.367E−07 −6.490E−07 1 4−2.593E−06 −1.397E−06 −1.009E−06   1.583E−06 6 0 −1.362E−10   6.378E−09−4.016E−11   1.554E−07 4 2 −4.191E−09   3.278E−08 −1.236E−09   8.426E−072 4   6.092E−08   1.695E−07   1.796E−08   1.745E−06 0 6   1.304E−06  3.750E−07   3.845E−07   2.663E−06

The coefficient of the aspherical surface of the optical surface thatconfigures the projection lens of the projection fluoroscope in theapplication example 3 is shown in Table 12 below.

TABLE 12 ASP1 ASP2 ASP3 ASP4 K −1 −1 −1 −1 B4 −2.458E−04 −4.033E−04  9.838E−04 −1.531E−03 B6 −1.012E−05 −5.142E−05 −7.720E−05   6.870E−05B8   9.372E−08   1.498E−06   1.466E−06 −1.997E−06

In Table 12 described above, the symbols K and Bi indicate thecoefficients for specifying an aspherical surface out of the aspericalsurfaces APS1 to APS4 which are the lens surfaces of two lenses 31 and32 that configure the projection lens 30.

FIG. 11 is a cross-sectional diagram of the projection fluoroscope 70 inthe application example 3. The prism 10 of the projection fluoroscope 70includes the first surface S11 that has a relatively weak negativerefractive power, the second surface S12 that has a relatively strongpositive refractive power, the third surface S13 that has a relativelyweak positive refractive power, the fourth surface S14 that has arelatively strong positive refractive power. The projection lens 30includes a first lens 31 that has a negative refractive power, a secondlens 32 that has a positive refractive power. The detail specificationof the optical system in the application example 3 will be described. Ahorizontal angle of view is 20.1°, a vertical angle of view is 11.4°, asize of the display area of the image display element is 9.22×5.18 mm, adiameter of a pupil is 5 mm, and a focal length is approximately 26 mm.

FIGS. 12A to 12F and FIGS. 13A to 13F illustrate the aberrations in theapplication example 3. Specifically, FIGS. 12A and 12B indicates theaberration in Y and X directions in the direction of 10° in the Xdirection and 5.7° in the Y direction, and FIGS. 12C and 12D indicatethe aberration in Y and X directions in the direction of 0.0° in the Xdirection and 5.7° in the Y direction, and FIGS. 12E and 12F indicatethe aberration in Y and X directions in the direction of −10° in the Xdirection and 5.7° in the Y direction. FIGS. 13A and 13B indicate theaberration in Y and X directions in the direction of 10° in the Xdirection and 0.0° in the Y direction, FIGS. 130 and 13D indicate theaberration in Y and X directions in the direction of 0.0° in the Xdirection and 0.0° in the Y direction, and FIGS. 13E and 13F indicatethe aberration in Y and X directions in the direction of −10° in the Xdirection and 0.0° in the Y direction.

Application Example 4

Data for optical surface that configures the prism and the projectionlens of the projection fluoroscope in the application example 4 is shownin Table 13 below.

TABLE 13 No Type R T Nd Vd 1 SPH ∞ 20.00 2 FFS1 −254.361 5.50 1.52555.95 3 FFS2 −79.045 −5.50 1.525 55.95 4 FFS1 −254.361 10.00 1.525 55.955 FFS3 −264.959 −10.00 1.525 55.95 6 FFS1 −254.361 10.00 1.525 55.95 7FFS3 −264.959 −31.00 1.525 55.95 8 FFS4 13.340 −18.00 9 ASP1 −9.698−5.00 1.525 55.95 10 ASP2 12.061 −10.88 11 SPH ∞ −1.44 1.458 67.82 12image surface

Regarding the optical surface of the prism that configures theapplication example 4, the inclination angle (tilt) TLY of the opticalaxis in the cross-section surface thereof, and the amount of shift(de-centered) DCX of the optical axis is shown in Table 14 describedbelow.

TABLE 14 No Type TLY (front) DCX (back) TLY (back) 2 FFS1 0 0 0 3 FFS2−28 0 28 4 FFS1 0 0 0 5 FFS3 0 0 0 6 FFS1 0 0 0 7 FFS3 0 48.49 −22.7 8FFS4 0 0 0

Regarding each optical surface of the prism that configures theapplication example 4, the coefficient expressed by the expandedpolynomial of the free-curved surface is shown in Table 15 below. Here,in Table 15, the symbols m and n mean the variables or the orders in thecoefficient Ak_(m,n). In addition, the symbol FFSk (k=1 to 4) means thek_(th) surface among the first to fourth surfaces S11 to S14 which arethe free-curved surfaces.

TABLE 15 m n FFS1 FFS2 FFS3 FFS4 2 0 −1.966E−03 −6.325E−03 −1.887E−03  3.748E−02 0 2 −1.439E−02 −9.557E−03 −1.301E−02   5.387E−02 3 0−1.259E−04 −1.056E−04 −8.537E−05   1.565E−04 1 2   4.023E−04   1.745E−04  2.729E−04   2.922E−04 4 0   6.777E−07   3.334E−06   3.482E−07−1.129E−04 2 2 −9.397E−06 −1.238E−05 −4.829E−06 −2.235E−04 0 4−2.372E−05 −9.077E−06 −1.219E−05 −1.358E−04 5 0   1.131E−08 −5.112E−08  4.404E−09 −1.139E−06 3 2   3.609E−07   9.136E−07   1.405E−07−4.142E−07 1 4   5.711E−07   1.088E−07   2.223E−07   3.262E−07 6 0−6.428E−11 −4.451E−10 −1.896E−11   2.125E−07 4 2 −3.893E−09 −5.405E−08−1.148E−09   3.505E−07 2 4 −5.244E−09   7.668E−09 −1.546E−09   1.388E−060 6 −1.294E−07   1.147E−08 −3.816E−08   1.254E−06

The coefficient of the aspherical surface of the optical surface thatconfigures the projection lens of the projection fluoroscope in theapplication example 4 is shown in Table 16 below.

TABLE 16 ASP1 ASP2 K −1 −1 B4 −7.866E−05 −1.023E−03 B6   1.386E−05  6.677E−05 B8 −2.029E−06 −3.272E−06

In Table 16 described above, the symbols K and Bi indicate thecoefficients for specifying an aspherical surface out of the aspericalsurfaces APS1 to APS2 which are the lens surfaces of a lens 31 thatconfigure the projection lens 30.

FIG. 14 is a cross-sectional diagram of the projection fluoroscope 70 inthe application example 4. The prism 10 of the projection fluoroscope 70includes the first surface S11 that has a relatively weak negativerefractive power, the second surface S12 that has a relatively strongpositive refractive power, the third surface S13 that has a relativelyweak positive refractive power, and the fourth surface S14 that has arelatively strong positive refractive power. The projection lens 30includes a first lens 31 that has a positive refractive power. Thedetail specification of the optical system in the application example 4will be described. A horizontal angle of view is 20.1°, a vertical angleof view is 11.4°, a size of the display area of the image displayelement is 9.22× 5.18 mm, a diameter of a pupil is 5 mm, and a focallength is approximately 26 mm.

FIGS. 15A to 15F and FIGS. 16A to 16F illustrate the aberrations in theapplication example 4. Specifically, FIGS. 15A and 15B indicates theaberration in Y and X directions in the direction of 10° in the Xdirection and 5.7° in the Y direction, and FIGS. 15C and 15D indicatethe aberration in Y and X directions in the direction of 0.0° in the Xdirection and 5.7° in the Y direction, and FIGS. 15E and 15F indicatethe aberration in Y and X directions in the direction of −10° in the Xdirection and 5.7° in the Y direction. FIGS. 16A and 16B indicate theaberration in Y and X directions in the direction of 10° in the Xdirection and 0.0° in the Y direction, FIGS. 16C and 16D indicate theaberration in Y and X directions in the direction of 0.0° in the Xdirection and 0.0° in the Y direction, and FIGS. 16E and 16F indicatethe aberration in Y and X directions in the direction of −10° in the Xdirection and 0.0° in the Y direction.

In Table 17 below, numeric data related to conditional expressions (1)to (3) with regard to each of the application examples 1 to 4 issummarized. In this case, the coefficients below satisfy any of theconditional expressions (1) to (3).

TABLE 17 Application Application Application Application Example 1Example 2 Example 3 Example 4 A1_(2.0) −1.114E−03 −6.341E−03 −3.944E−03−1.966E−03 A1_(0.2) −1.673E−02 −1.288E−02 −1.423E−02 −1.439E−02 A3_(2.0)−1.070E−03 −6.088E−03 −3.786E−03 −1.887E−03 A3_(0.2) −1.505E−02−1.194E−02 −1.299E−02 −1.301E−02 A1_(2.0)-A1_(0.2) −1.784E−02 −1.922E−02−1.817E−02 −1.635E−02 A3_(2.0)-A3_(0.2) −1.612E−02 −1.803E−02 −1.678E−02−1.489E−02 A1_(2.0)-A1_(0.2)   1.561E−02   6.539E−03   1.028E−02  1.242E−02 A1_(2.0)-A3_(2.0) −4.458E−05 −2.536E−04 −1.578E−04−7.863E−05 A1_(0.2)-A3_(0.2) −1.679E−03 −9.382E−04 −1.234E−03 −1.379E−03

In addition, in Table 18 below, with regard to each of the applicationexample 1 to 4, numeric data related to a gap between the first surfaceS11 and the third surface S13 and an inclination angle of the secondsurface S12 with respect to the first surface S11 are summarized.

TABLE 18 Application Application Application Application Example 1Example 2 Example 3 Example 4 Gap between 10.00 10.00 10.00 10.00 S11and S13 (mm) Angle of 28 28 28 28 inclination of S12 to S11 (°)

In addition, the visibility of the prism with respect to the externallight is influenced according to the conditional expression (3) asfollows. When the thickness of the prism is T and the refractive indexis N (=Nd (each application example)), the visibility Dx in the x axisdirection on the optical axis of the prism and the visibility Dy in they axis direction are influenced according to:

Dx=2000(N−1)(A1_(2,0) −A3_(2,0)+(2T(N−1)/N)×A1_(2,0) ×A3_(2,0))

Dy=2000(N−1)(A1_(0,2) ×A3_(0,2)+(2T(N−1)/N)×A1_(0,2) ×A3_(0,2)).

Based on above expression, numeric data related to the visibility ineach application example 1 to 4 is summarized in Table 19.

TABLE 19 Application Application Application Application Example 1Example 2 Example 3 Example 4 Visibility −0.04 0.01 −0.06 −0.06 Dx (D:Diopter) Visibility 0.06 0.13 0.04 −0.10 Dy (D: Diopter) Thickness 10 1010 10 of prism (mm) Refractive 1.525 1.525 1.525 1.525 index N

Others

The invention is described with reference to each embodiment above.However, the invention is not limited to the embodiments describedabove, a variety of embodiments can be implemented without departingfrom the scope and spirit thereof.

In the description above, the half mirror layer 15 (transflective film)is formed in the horizontally long rectangular area. However, thecontour of the half mirror layer 15 can be suitably changed according tothe usage or other uses. In addition, the transmittance and thereflectance of the half mirror layer 15 can also be changed according tothe usage or others.

In the description above, a distribution of the display brightness isnot particularly adjusted in the image display element 82. However, in acase where a difference in the brightness occurs according to theposition, the distribution of the display brightness can be adjusted tobe uneven.

In the description above, the image display element 82 made from atransmission type liquid crystal display device or the like is used asthe image display device 80. However, various devices can be used as theimage display device 80 without being limited to the image displayelement 82 made from a transmission type liquid crystal display deviceor the like. For example, a reflection type liquid crystal displaydevice can be used to configure the image display device 80.Alternatively, instead of the image display element 82 made from atransmission type liquid crystal display device or the like, a digitalmicro mirror device can also be used. In addition, a self-emittingelement represented by such as an LED array or an OLED (organic EL) canalso be used as the image display device 80.

In the description above, the image display device 80 made from thetransmission type liquid crystal display device or the like. However,instead of this, a scanning type image display device can also be used.

Specifically, as illustrated in FIG. 17, the first display device 100Aas a virtual image display device includes a light guide section 20 andan image display device 380. The light guide section 20 corresponds tothe first optical portion 103 a in FIG. 1 in the embodiment describedabove. That is, since the light guide section 20 corresponds to thecombination of the prism 10 and the light transmitting member 50, thedescription will be omitted here. The image display device 380 is adevice that forms an intensity-modulated signal light and emits thesignal light as a scanning light TL, and includes a signal light formingsection 381 and a scanning optical system 382.

The signal light forming section 381 includes a light source, and emitsa signal light LL which is modulated and formed based on a controlsignal from a not-illustrated control circuit. The scanning opticalsystem 382 scans and emits the signal light LL that passed through thesignal light forming section 381. Here, the scanning optical system 382is formed of a MEMS mirror or the like, and synchronizes the signallight LL from the signal light forming section 381 to the modulation tochange the posture, and adjusts the light path of the signal light LL,and then performs a two dimensional scanning to change the emissionangle of the light (scanning light TL) in horizontal and vertical. Inthis way, the image display device 380 causes the scanning light TLwhich is to be the image light GL to be incident on the light guidesection 20, and to be scanned with respect to all the partial area inwhich the half mirror layer 15 of the second surface S12 is formed.

The operation of the illustrated first display device 100A will bedescribed. The image display device 380 emits the signal light LL towardthe fourth surface S14 of the light guide section 20 as the scanninglight TL as described above. The light guide section 20 internallyguides the scanning light TL which passed through the fourth surface S14by total reflection and the like, to arrive at the half mirror layer 15.At this time, the scanning light TL is scanned on the surface of thehalf mirror layer 15, and then, a virtual image is formed by the imagelight GL as a trajectory of the scanning light TL. The eyes EY of thewearer of the device take the virtual image, and then the virtual imageis recognized. Moreover, in the illustrated case, the fourth surface S14which is the light incident surface among the light guide section 20 isvertical plane with respect to the optical axis of the scanning lightTL.

In addition, in the above-described embodiment, the device is configuredsuch that the prism 10 which is a light guide member and the lighttransmitting member 50 which is an auxiliary prism cover the entirefront of the eyes EY of the wearer, but is not limited thereto. Forexample, as illustrated in FIG. 18A and FIG. 18B, the device may beconfigured in such a manner that a part that includes the second surfaceS12 which has a curved surface shape having the half mirror layer 15covers only a part of eyes EY, that is, covers a part of eye front,thus, the device may be configured in such a small type that there alsoexists an uncovered part. In addition, in this case, even with aconfiguration in which a total reflective mirror instead of the halfmirror layer 15 is disposed, by making the prism 10 and the lighttransmitting member 50 sufficiently small, the outside can be observedfrom the periphery of the prism 10 and the light transmitting member 50,without observing by the see-through. In addition, in the illustratedcase, the half mirror layer 15 is formed on the entire or substantiallyentire surface of the second surface S12. However, the half mirror layer15 may be formed on only a part of the second surface S12. In addition,in the example in FIG. 18B, the half mirror layer 15 is disposed onsubstantially front of the eyes EY. However, the half mirror layer 15may be disposed on a position shifted from the front of the eyes EY tomake the image visible by moving the line sight. For example, theposition of the eyes EY may be somewhat lowered (the position of theprism 10 and the light transmitting member 50 may be somewhat raised).In this case, for example, the lower half of the eyes EY is in a stateto be seen from the bottom of the prism 10 and the light transmittingmember 50.

In the above description, the virtual image display device 100 thatincludes a pair of display devices 100A and 100B is described, but asingle display device may also be included. That is, it is not necessaryto provide sets of the projection fluoroscope 70 and the image displaydevice 80 corresponding to both of right eye and left eye. Theprojection fluoroscope 70 and the image display device 80 only for anyone of the right eye or left eye may be provided, and then the devicemay be configured to see the image in a single-eye view.

In the above description, a gap between the pair of display device 100Aand 100E in the X direction is not described. However, the gap betweenthe two display devices 100A and 100B is not limited, and it is possibleto adjust the gap by a machine or mechanism. That is, it is possible toadjust the gap between the two display devices 100A and 100B in the Xdirection according to the eye width of the wearer.

In the above description, the half mirror layer 15 is merely atransflective film (for example, a metal reflective film or a dielectricmultilayer film). However, the half mirror layer 15 can be replaced by aplane or a curved hologram element.

In the above description, a detail description is performed under theassumption that the virtual image display device 100 is a head-mountingdisplay. However, the virtual image display device 100 can be modifiedto a head-up display.

In the above description, in the first surface S11 and the third surfaceS13 of the prism 10, the image light is totally reflected and guided bythe boundary of the surface and the air without applying a mirror or ahalf mirror on the surfaces. However, the total reflection in thevirtual image display device 100 according to the invention includes thereflection reflected at the mirror coating or a half mirror film formedon the entire or a part of the first surface S11 or the third surfaceS13. For example, a case is included, in which the incident angle of theimage light satisfies the total reflection conditions, and the mirrorcoating is applied to entire or a part of the first surface S11 or thethird surface S13, and actually all of the image light is reflected. Inaddition, if the image light with a sufficient brightness can beobtained, the entire or a part of the first surface S11 or the thirdsurface S13 may be coated by a somewhat transmissive mirror.

In the above description, the prism 10 is laterally extended in linewith the eyes EY. However, the prism 10 can be disposed so as to bevertically extended. In this case, the device may have a structure inwhich the optical member 110 is disposed in parallel, not in series.

The entire disclosure of Japanese Patent Application No. 2013-025271,filed Feb. 13, 2013 is expressly incorporated by reference herein.

What is claimed is:
 1. A virtual image display device for recognizing animage light and an external light at the same time, the devicecomprising: an image element that generates an image light; and a prismthat includes three or more non-axisymmetric curved surfaces, and inwhich an intermediate image is formed as a part of an optical system,wherein, when the external light passes through a first surface and athird surface among a plurality of surfaces that configure the prism torecognize the outside, a visibility is substantially zero, wherein thefirst surface and the third surface form a concave surface shape withrespect to an observer side, and wherein the image light from the imageelement is totally reflected at the third surface, totally reflected atthe first surface, totally reflected again at the third surface, totallyreflected again at the first surface, and reflected at the secondsurface, and then transmits through the first surface, and arrives atthe observer side.
 2. The virtual image display device according toclaim 1, wherein, with an origin of each surface which configures theoptical system to be a reference, when an expression of a surface shapeis polynomially expanded with respect to an orthogonal coordinates x andy which are extended in a tangential direction from the origin, theconditions in the below-described expressions (1) to (3) are satisfied,with coefficients of the terms x^(m)·y^(n) of the polynomial whichindicates the k_(th) surface as Ak_(m,n)−5×10² <A1_(2,0) +A1_(0,2)<−1×10⁻³ and −5×10⁻² <A3_(2,0)+A3_(0,2)<−1×10⁻³  (1)|A3_(2,0) −A3_(0,2)|<5×10⁻²  (2)|A1_(2,0) −A3_(2,0)|<5×10⁻³ and |A1_(0,2) −A3_(0,2)|<5×10⁻³  (3)
 3. Thevirtual image display device according to claim 1, wherein a half mirroris formed on the second surface and the image light is presented to theobserver, and wherein a light transmitting member is integrally disposedoutside of the second surface and the visibility with respect to theexternal light is substantially zero, and the external light and theimage light are presented to the observer in overlapping.
 4. The virtualimage display device according to claim 3, wherein the lighttransmitting member includes a first transmitting surface and a secondtransmitting surface in the observer side, and a third transmittingsurface in the external side, wherein the second surface of the prismand the second transmitting surface of the light transmitting memberhave substantially the same curved surface shapes, and wherein thesecond surface and the second transmitting surface are integrated. 5.The virtual image display device according to claim 1, furthercomprising: a projection lens that causes the image light from the imageelement to be incident on the prism, wherein at least a part of theprism and the projection lens configure a relay optical system thatforms an intermediate image.
 6. The virtual image display deviceaccording to claim 5, wherein the projection lens is formed of anaxisymmetric lens and includes at least one or more aspherical surfaces.7. The virtual image display device according to claim 5, wherein theprism includes a fourth surface that is disposed to face the projectionlens and causes the image light emitted from the projection lens to beincident and guides the image light to the third surface.
 8. The virtualimage display device according to claim 1, wherein the prism includes afirst prism portion of the light emitting side including the firstsurface, the second surface, and the third surface, and a second prismportion of the light incident side, and wherein the first prism portionand the second prism portion are integrally formed.
 9. The virtual imagedisplay device according to claim 8, wherein the second prism portionincludes at least one or more optical surface, and wherein anintermediate image is formed by the image element, the projection lens,and at least a part of the prism where the second prism portion isincluded.
 10. The virtual image display device according to claim 9,wherein the image element is an image display element that emits animage light from the display position, and wherein the projection lensand at least a part of the prism where the second prism portion isincluded cause the image light emitted from the display position of theimage display element to form an image in the prism to form theintermediate image, as the relay optical system.
 11. The virtual imagedisplay device according to claim 9, wherein, in the third surface, thefirst prism portion includes a first region where the image light passedthrough the second prism portion is totally reflected in a first time,and a second region where the image light is totally reflected in asecond time, and wherein the intermediate image is formed by theprojection lens and a part where the first region of the second prismportion and the first prism portion is included.
 12. The virtual imagedisplay device according to claim 11, wherein the first prism portionand the second prism portion cause the intermediate image to be formedin front or back of the first region among the third surface in a stateof folded back.
 13. The virtual image display device according to claim8, wherein the second prism portion includes a fourth surface that isdisposed to face the projection lens and causes the image light emittedfrom the projection lens to be incident on and guides to the thirdsurface, and a fifth surface that is interposed the fourth surface withthe third surface of the first prism portion.
 14. The virtual imagedisplay device according to claim 1, wherein the gap between the firstsurface and the third surface is equal to or more than 5 mm and is equalto or less than 15 mm.
 15. The virtual image display device according toclaim 1, wherein an inclination angle of the second surface with respectto the first surface is equal to or more than 20° and is equal to orless than 40°.
 16. The virtual image display device according to claim1, wherein, when the device is mounted on, the optical system thatincludes the prism covers a part of the front of the observer's eyes,and a part where the front of the eyes is not covered exists.
 17. Thevirtual image display device according to claim 1, wherein the imageelement includes a signal light forming section that emits a signallight which is modulated according to the image, and a scanning opticalsystem that emits the signal light as a scanning light by scanning thesignal light incident on from the signal light forming section.